This disclosure relates to a method and apparatus for supporting and retaining a fan shaft in a gas turbine engine.
A gas turbine engine includes a fan section with a fan shaft that is mounted on bearings to rotate fan blades. The fan shaft is rotatable about an axis and the bearings include a fore bearing and an aft bearing that is spaced rearwardly from the fore bearing along the axis. A bearing support structure supports outer races of the fore and aft bearings, while respective inner races are supported on the fan shaft. A bearing spacer is fixed to the fan shaft and is positioned between inner races of the fore and aft bearings.
To adequately support the fan shaft, an axial preload in excess of a highest predicted windmill reaction load should be provided to prevent skidding of a main thrust bearing. Skidding can occur when a high reverse load is applied during a windmilling operation on-wing, or when windmilling while an aircraft is parked on the ground. Windmilling refers to a condition where the gas turbine engine is in an unpowered mode, i.e. is not supplied with fuel, and ambient air is drawn or driven into the gas turbine engine causing the fan blades to rotate at a relatively low rotational frequency.
A support and retention system for the fan shaft and bearings should allow windmill operation in excess of normal windmill loads without unloading the main thrust bearing such that the bearing would skid. Excessive windmill loads could occur during fan blade-out or a high “g” load maneuver, for example.
Further, in addition to accommodating windmilling conditions, the support and retention system for the fan shaft and bearings should maintain a nearly constant preload over a full range of static and dynamic loading, as well as for all thermal conditions.
One known support and retention system utilizes a plurality of coil springs to provide the axial preload. The coil springs are mounted directly to the bearing support structure using various mounting hardware. As many as twenty-four different coil springs can be required to provide sufficient axial preload. This requires a significant amount of mounting hardware, which is subject to wear and can adversely affect the overall fatigue life of the system. Also, the significant number of parts is disadvantageous from a maintenance cost and labor perspective.
Another disadvantage with the current support and retention system is that the arrangement of the series of coil springs and associated stacked components provides manufacturing tolerance challenges. Accordingly, there is a need to provide a support and retention system that can accurately set a bearing preload without requiring excessive manufacturing tolerances, as well as addressing the other short comings discussed above.